For goals like a higher speed of operation and a saving of power consumption of microelectronic devices, the challenge to higher integration of large-scale integrated circuits continues. To meet increasing demands for shrinkage of circuit patterns, the advanced semiconductor microprocessing technology becomes important. For example, the technology for shrinkage of circuit-constructing wiring patterns and the technology for shrinkage of contact hole patterns for cell-constructing inter-layer connections become essential.
The advanced microprocessing technology relies on the photolithography using photomasks. The photomask is one important area of the miniaturization technology as are the lithography system and resist material. For the purpose of obtaining a photomask having a fine-size wiring pattern or fine-size contact hole pattern, efforts are made to develop the technique of forming a more fine and accurate pattern on a photomask blank.
In order to form a high accuracy photomask pattern on a photomask substrate, it is of first priority to pattern a resist film on a photomask blank at a high accuracy. Since the photolithography for microprocessing semiconductor substrates employs reduction projection, the size of a pattern formed on a photomask is about 4 times the size of a pattern formed on a semiconductor substrate. This does not mean that the accuracy of the pattern formed on the photomask is accordingly loosened. It is necessary that the photomask pattern be formed at a high accuracy.
At the present, the size of a circuit pattern written on a semiconductor substrate by photolithography is far smaller than the wavelength of exposure light. If reduction exposure is carried out using a photomask having a pattern which is a mere 4-time magnification of the circuit pattern, the photomask pattern is not faithfully transferred to the resist film due to interference of exposure light and other impacts.
Super-resolution masks addressing the problem include OPC masks in which the so-called optical proximity correction (OPC), i.e., the technology for correcting the optical proximity effect of degrading transfer properties is applied to photomasks and phase shift masks which cause a phase shift of 180° to exposure light transmitted by the pattern, to establish a sharp intensity distribution of incident light. For example, in some OPC masks, an OPC pattern (hammer head, assist bar or the like) having a size of less than half of a circuit pattern is formed. The phase shift masks include halftone, Levenson and chromeless types.
In general, a mask pattern is formed by starting with a photomask blank having a light-shielding film on a transparent substrate, forming a photoresist film on the photomask blank, exposing the photoresist film to light or electron beam (EB) to write a pattern, and developing the photoresist film to form a photoresist pattern. Then, with the photoresist pattern made etching mask, the light-shielding film is etched or patterned to form the photomask pattern. For obtaining a fine photomask pattern, it is effective to reduce the thickness of a photoresist film (i.e., thinner resist film) for the following reason.
If only a resist pattern is shrunk without reducing the thickness of a resist film, the resist pattern feature functioning as the etching mask for the light-shielding film has a higher aspect ratio (ratio of resist film thickness to feature width). In general, as the aspect ratio of resist pattern features becomes higher, the pattern profile is more likely to degrade. Then the accuracy of pattern transfer to the light-shielding film is reduced. In extreme cases, the resist pattern partially collapses or strips off, resulting in pattern dropouts. In association with the shrinkage of a photomask pattern, it is necessary that the resist film used as the etching mask during patterning of a light-shielding film be thinned to prevent the aspect ratio from becoming too high. An aspect ratio of up to 3 is generally recommended. To form a resist pattern having a feature width of 70 nm, for example, a resist film thickness of up to 210 nm is preferable.
On the other hand, in the ArF lithography using a photomask and ArF excimer laser as exposure light, the photomask pattern is transferred to a processable substrate, typically a photoresist film on a semiconductor wafer. Under the current advance of the miniaturization technology, the pattern width (on-wafer size) is less than 100 nm for standard products and less than 20 nm for advanced products. The minimum width of a main pattern on the photomask that complies with the reduced pattern width is about 100 nm, and the minimum width of an auxiliary pattern is reduced below 100 nm (specifically, about 70 nm) as a result of complication of OPC.
For the light-shielding film which is etched using the pattern of photoresist as an etching mask, a number of materials have been proposed. In particular, neat chromium films and chromium compound films containing chromium and at least one of nitrogen, oxygen and carbon are generally used as the light-shielding film material. For example, Patent Documents 1 to 3 disclose photomask blanks wherein chromium compound films are formed as the light-shielding film having light shielding properties necessary for the photomask blank for use in ArF excimer laser lithography.
For the fabrication of photomasks, the exposure method using electron beam (EB) is the mainstream of resist patterning. For EB emission, a high accelerating voltage of 50 keV is employed in order to enable further miniaturization. While there is a tendency that the resist reduces its sensitivity in order to achieve a higher resolution, in the EB lithography system, the current density for EB emission experiences a remarkable leap from 40 A/cm2 to 800 A/cm2 from the aspect of productivity enhancement.
When EB is directed to an electrically floating photomask blank, electrons accumulate on the surface of the photomask blank to charge it at a negative potential. An electric field due to the electric charge causes the EB trajectory to be bent, resulting in a low accuracy of writing position. To avoid such fault, the EB lithography system adapted for high energy/high density EB writing is designed such that EB writing is performed with the photomask blank being grounded. For example, Patent Document 4 discloses an earth mechanism for grounding a photomask blank using an earth pin.
However, if ground resistance is significant, the potential at the photomask blank surface increases by the product of ground current and ground resistance, and the accuracy of writing position is accordingly reduced. If EB writing is performed in the state where the ground resistance is very high, an abnormal discharge or substrate failure can occur within the imaging vacuum chamber, causing contamination to the system. It is thus important to acquire a sufficient ground resistance, suggesting that a grounding method with a low ground resistance is required for the EB lithography system, and the photomask blank must have a sufficient conductivity.